(1) Field of the Invention
The present invention relates to an antivibration suspension device for a mechanical element, and to an aircraft having such a device.
The invention thus lies in the narrow technical field of devices for reducing vibration on board aircraft.
(2) Description of Related Art
Aircraft include in particular rotorcraft that have at least one lift rotor that is connected to a carrier structure, which carrier structure is commonly referred to as an “airframe” or as a “fuselage”.
Such an aircraft also includes a power plant driving a main gearbox (MGB) secured to the carrier structure of the aircraft. The main gearbox then includes a mast for driving the lift rotor in rotation.
It should be observed that the main gearbox is often connected to the carrier structure via a bottom wall and auxiliary fastener means generally comprising three or four sloping holder bars. The structure for mounting the lift rotor and including the main gearbox and the holder bars is sometimes referred to as a “pylon” by the person skilled in the art, given its shape.
The lift rotor and the main gearbox form a mechanical assembly that can give rise to vibration liable to lead to discomfort for the occupants of the aircraft, by generating vibratory motion and noise. Furthermore, such vibration puts constraints on the equipment of the aircraft that is located in the carrier structure.
In order to provide a good level of comfort in the aircraft, it is advantageous to incorporate an antivibration system as close as possible to the source of the dynamic forces that give rise to the vibration. Installing an antivibration system between the mechanical assembly and the carrier structure is advantageous. Installing the system in that way enables all of the torsor components exciting the lift rotor to be processed without presenting the complexity and the constraints involved with incorporating the system in the rotary portion of the lift rotor, given that it is the rotor that is the main generator of vibration.
Under such circumstances, various main gearbox suspension devices are known for the purpose of at least reducing vibration within the aircraft, and in particular within a cabin for the comfort of pilots and of passengers.
Such a suspension device must be capable firstly of transmitting the static loads transmitted by the mechanical assembly, and secondly of filtering the vibration that is induced by the rotor. It is observed that this vibration is produced at very low frequency and is therefore uncomfortable for the occupants of the aircraft.
Furthermore, it can be understood that a manufacturer seeks to obtain a suspension device that is of minimum impact in terms of weight and cost.
It should also be observed that certain aircraft have a suspension rotor that may rotate in flight at various speeds of rotation. It is then advantageous to obtain a suspension device that matches that type of technology.
In the state of the art, there is Document FR 2 474 996, which discloses a suspension system for a mechanical assembly of a helicopter.
That suspension system has four sloping holder bars and a suspension plate for a main gearbox. The suspension plate has a central portion secured to a bottom of the main gearbox and one arm per sloping holder bar. Each arm is hinged in the region of its proximal end firstly to the fuselage and secondly to an extreme zone of the corresponding holder bar. The distal end of each bar also carries a flapping mass (“masse battante” in French language) that stresses the arm in bending.
That suspension system is very effective, but its adjustment depends on the relative proportions between the components of the force torsor acting on the rotor head, such that it is not optimally adjusted over the entire flight spectrum of the aircraft. In addition, the behaviors of the flapping masses are mutually coupled, which means that a search for optimum adjustment is lengthy and iterative.
Also known are Documents U.S. Pat. Nos. 4,311,213 and 4,405,101.
According to Document FR 2 747 098, a suspension system has flapping masses and means for moving those flapping masses in a longitudinal direction along a support.
Under such circumstances, the device has at least one sensor suitable for measuring the values of at least one parameter representative of a performance criterion of the device and a computer receiving said values in order to further control the positions of the flapping masses via their respective movement means.
That device can therefore combat vibration being produced at varying frequencies. Nevertheless, the system behaves as a passive system, of adjustment that can be adapted under quasi-steady conditions.
Document FR 2 982 583 describes an antivibration suspension system having at least one holder bar hinged via a bottom end to a lever of a suspension means. The lever extends from a distal end supporting at least one flapping mass to a proximal end hinged to a carrier structure. The suspension system includes torsion return means provided with a rotary actuator for matching the stiffness in torsion of the lever to flight conditions.
Document U.S. Pat. No. 5,219,143 discloses a system having actuators within holder bars of a main gearbox. A computer issues orders to the actuators on the basis of measured forces or accelerations. The dynamic forces that need to be generated are high, and involve the use of actuators that are heavy and expensive, and possibly difficult to maintain.
Document U.S. Pat. No. 6,467,723 describes a vibration control system having actuators arranged in housings formed in the fuselage. The drawback of that type of system is that vibration is processed locally and generally along a single axis (vertical or lateral).
Document US 2012/0292434 describes a system for actively controlling vibration, which system is implemented on the hub of a rotor. It enables all of the components of the torsor to be processed, and its drawback lies in the complexity of being installed on the hub.
Document FR 2 784 350 describes a system that is remote from the field of the invention for the purpose of reducing the vibration caused by the flow of air passing through a rotor and impacting against a structure. The processing is generally local and along only one direction.
Document US 2013/0233998 proposes a system having an actuator for adjusting an antiresonance frequency. The actuator generates a static force to move a mass for that purpose.
Also known are Documents US 2011/027081, FR 2 795 386, and EP 0 488 845.